Systems for electronically monitoring changes of physical documents have already been developed that entail attaching electronic files to physical objects, by putting the electronic files on removable media such as disks, tapes, or CD's and then attaching the removable media to the physical object using a clip or using a common folder, stack or other container. Some disadvantages with this solution are that the electronic attachment is bulky, may be accidentally detached, and cannot be accessed remotely or through a network (e.g., one could not include the electronic files in network-wide searches when stored this way).
Another solution is to virtually attach some code to the electronic file indicating where the physical object is supposed to be filed. This type of system is analogous to an electronic catalog of library books. Some disadvantages of this solution are that the object might not be where it is “supposed” to be filed, and moreover, this type of solution does not empower the user to use the physical object to find the electronic file. To solve the latter problem, one might attach a machine-readable physical tag, such as a Radio Frequency Identification (RFID) or barcode, to the physical document, so that the electronic files are invoked when the tag is detected. Disadvantages of this approach include the cost of tagging the objects and that either the object is defaced or the tags can accidentally become detached. That would still leaves the first problem, which is that the object is not where it is supposed to be. Additional problems with this type of solution, as applied to an office environment, is that the process of updating the library-type database would be very labor-intensive, especially if the changing content of office documents is also to be recorded in the electronic database.
Both fluorescent and phosphorescent inks are well known in the art. See, for example, Sarada et al. (U.S. Pat. No. 5,569,317); Houle (U.S. Pat. No. 6,513,921); Auslander (U.S. Pat. No. 5,684,069). Both fluorescent and phosphorescent inks are luminescent, the difference being that the latter continue to glow for a time after the stimulating source has been removed. Neither of these types of inks, nor any other comparable identifying technique, has been utilized in conjunction with cameras to monitor objects such as documents at various locations as functions of time, or to coordinate those physical objects with electronic versions thereof.